Automatic frequency control system



' June 3o, 1942. A, F, PQMEROY 2,288,025A

AUTOMATIC FREQUENCY CONTROL SYSTEM Filed Aug. 3, 1940 fm In Im :In

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r/efa. nerf/www0 Uril I /A//EN TOR my @f qa/MW), ,4.5 ,DOME/Por aT/mw ATTORNEY- Patented June 30, 1942 UN'TED STATES PTENT 27,288,025 l AUTOMATIC FRQUENCY CoNTRoL sisTEir Auen F. Pomeroy, Long xslandoity, assisi-.

or to Bell Telephone Laboratories,l Incorporated, New York, N. Y., a corporato'i f New York Application August 3,

ity of two waves, the wave from the controlled source' and the Wave from the controlling source, to a degree measurable only in terms of phase difference; In a particular embodiment of the invention, theV principle is used to maintain the difference between the frequencies of the wavesAV from two oscillators equal to the frequency of a wave from a third, that is controlling, oscillator to the degree aboveindicated.

The principal object of the invention is to achievethe automatic frequency control of one oscillator by another oscillator to an extent not realizable by prior art means, and consistently with comparatively great simplicity of circuit and economy of plant.

A` secondary object of the invention is to'inaintain with an extreme degree of fidelity the difference in frequency of the waves from two oscillators equal to thefr'equency of the wave'frorn a third oscillator.

Having in mind the principal object of the invention, and especially the characteristically extremely close'frequenc'y correspondence of the two waves concerned, it is a subsidiaryfobject of the invention to utilizein the operation of the control principle the phenomenon of an increasing phase difference which'initiates a changeV of relative frequency. This principle, by inherency, te'rnlsto'resultv in the maximum possible sensitivity and responsiveness of control. The

means of the invention translates" this' relativeA phase change into a corresponding change of potential which is then used, through intermediary devices of the invention, to control the frequency of the controlled source.

As the invention is carried out in a preferred embodiment, the waves from the two sources concerned are impressed on a lconjugate input phase detector, in the normal operation'of which,

that is, when there is exactV correspondence be-Vv determining circuit. Since the effect, that is the control, is responsive to the initiation Vof 1940, seal No. asojzs the relativechange frequency and since this effect tendsto change thefrequency in such a direction as to tend t o restore the equality relationship, the conditions are present for a continuous and extremely sensitive frequency control.

The frequency control bythe unbalanced potential is achieved, in a preferredformof the invention, by theuseA of what has some times beendenomi'nated l a Areantaric control tube Aprinciple whereinV a chan eV inicontolfelectrode potential of areactance'c" `rol .tube effects a change inirnpdanco cathode circuit therefore', .by the reection principle in 'thel iin-p' ance'fY the'out'put circuit ofthe reactanceicontrol tbe... 1n.lthe instant case' this' otbut'impdaee. change ,is a change control tube circitaslawhol ai pushpullcombination of which". would ,otherwiseappear as conventional reactancetube used aconventional way'. This `e e'de'nt is necessary' if a push-pull oscillator vis'to be" controlled by aY balancedto-ground rea'otance change; Apush-pull Oscillator'is usually ncessryH Vin order to obtainV high power. qut'put with low harmonie Content;

TheI principle of the; invntion may be lised.

alternatively, and as will be disclosed in the ex# ernplilcation'of the )invention to be describedin detail'later', to maintain .the difference in frequency of two unit.oscillatorsffexactly Vequal to the frequencylof athird oscillator which may be thought' of as thestandard or" controlling'oscillato. A. Ir'iftlifis.r instance. the' waVes vfrom the two nitoscill'ators are' combined, as 'by an electronic modulator, and the resultant Wave is used like the wave .fromrthe controlled oscillator in the generic caseA The control over the frequency of this' simulated generic controlled oscillator, and therefore; of the differeilce frequency in the output'of thefcombin'in'g device, is best achieved by operating'on 'one vof the'two unit'loscillators, the variation of frequency of the wave`v therefrom, of course, being reflected in the' difference frequency wave.

Thei` control circuitof the invention as de'- scribedirnnidiately" alcoveA has f been use'd with su'cces`s`in' envelopedelay measurements as' ap-` By the usel 0f a plied to circuits and apparatus for television transmission at frequencies ranging from 250 kilocycles to megacycles with a difference frequency of 50 kilocycles. An envelope delay may be defined as the slope of a phase versus frequency curve and, therefore, is a measure of the retardation as indicated by loss of phase, of the very high frequency currents used in this art in its progress through the circuits and apparatus concerned. The choice of difference frequency was determined by the necessity of having the frequency interval Af small enough so that the computed slope may be considered to be substantially the same as a `differential of the phase with respect to frequency. This difference frequency had to be xed to within at least two cycles per second, a standard diicult to be realized with other types of frequency control. It was found that the direct potential bias applied to thecontrol electrodes of the reaction tubes, and resulting from the slight unbalance of the phase detector incidental to the operation of the circuit, was adequate to control the frequency extremely accurately over a wide enough range' to cover the frequency drift of the oscillators. Since the two unit oscillators were of like type, the frequency drift tended to be in like directions so as to be mutually compensatory to a great extent.

At least two other applications of the frequency control circuit of the invention, especially in the form last described, readily present themselves, both applications involving the control of the oscillator frequency in demodulation. For instance in making phase delay measurements it is necessary to control rather closely the low frequency from the demodulator, this requiring a frequency control of the high order of merit represented by this invention. Likewise, in a wave analyzer the sensitivity may be increased as the width of the pass band in the low frequency system is decreased (this because of resistance noise) but such a narrow pass band requires extremely accurate frequency control in order that the beat frequency may stay in the band of the band-pass filter.

In the accompanying drawing:

Fig. 1 illustrates in highly diagrammatic form the relationship of the various units of the circuit of the invention as a whole;

Fig, 2 illustrates the conjugate input phase detector of the invention while Fig. 3 illustrates the same in a more diagrammatic form as an aid in the understanding of its mode of operation;

Figs. 4, 5 and 6 are electrical-geometricalgraphical devices which, taken together, illustrate the operation, under various conditions, of the phase detector of Figs. 2 and 3 and Fig. 7 illustrates the frequency control circuit immediately posterior to the. phase detector, or to an amplifier following the phase detector, together with the circuit of the unit oscillator directly controlled thereby.

The circuit of Fig. l, disclosing in diagrammatic form the circuit or system of the invention as a whole in its embodiment in which the principle of the invention is utilized to maintain the difference in frequency of the waves of the two unit oscillators exactly equal to the frequency of the wave from a third, the controlling or standard oscillator, requires very little description supplemental to the description above in the statement of the invention, because of the functional labeling of the parts. It should again be emphasized that this circuit, while disclosing what is perhaps a preferred embodiment of the invention as expressed in a specific form, just as well discloses the generic form of the invention as directed to a means for maintaining an exact equality of two frequencies whether one of these two frequencies is a prime source or is itself a composite of two component frequencies, this as has been explained in the statement of invention above. In the detailed description to follow, in the interest of simplicity as well as clarity of disclosure, the functional labeling of the elements in the drawing will be used to identify these elements instead of perhaps the more usual device of literal or numerical labels.

The oscillators Nos. I and 2, generating waves of frequencies f1 and f2, respectively, should preferably be duplicates so that their normal frequency drifts are the same. These oscillators are connected, through the mixer, to the high frequency detector. An amplifier may be used between either oscillator, or each oscillator, and the mixer. It is shown used between oscillator No. 2 and said mixer. As illustrated the output of the oscillators either individually or together may be taken from the output of the mixer or, which is the same thing, from the input of the high frequency detector. Obviously the separate outputs from said oscillators, may be taken directly from the oscillators, in-` stead of through the intermediary of the mixer.- The mixer is non-essential. It in no way com-` pares in function with a modulator or detector circuit. As its name implies it is merely an effective means for mixing or arithmetically combining the waves from the unit oscillators and conditioning them for impression on the high frequency detector from which the difference frequency, as a modulation product, is derived.

` Practically, the mixer serves the function of predetermining the relative amplitudes of the waves so impressed on the high frequency detector and, therefore, commonly consists of a ,resistance network, the resistors of which match impedances and, as above, condition the impressed waves as to their relative amplitudes. It might well comprise, besides resistances, an electronic tube adapted to function as an amplifier but if so the amplifier must be distortionless. As above suggested, the mixer may be dispensed with, the relative amplitudes being determined by operation of the respective unit oscillators themselves.

Since the high frequency detector, by its modulation or demodulation action, gives a difference frequency f1-f2, this means that the frequencies of the unit oscillators are relatively high. In a sense, therefore, the combination of unit oscillators and high frequency detector as a means of obtaining a fairly small frequency, 50 kilocycles in a practical case, is an exemplification of the use of the heterodyne principle to easily and efficiently achieve a, constant, but easily adjustable, low frequency by operation on one or other of said unit oscillators.

The amplifier disclosed as following the high frequency detector not only raises the difference frequency wave level but also tends to prevent reaction from oscillator No. 3, which is the controlling standard oscillator, on the high frequency detector output through the phase detector which is shown immediately following the amplifier. This phase detector is a means, somewhat analogous to a high frequency detector or demodalator forcombiningthe'difference frequencywave `and-the;V wavefromthe controlling oscillatorythatis,oscillator-No; 3. It-differs in function' from'such conventionalV detector that itis phaseresponsive so Ythat 'the output `product is expressed lin terms of a vdifference* inphase of the -two coniugately' impressed input waves. The sensitivity or `responsiveness' of"v the control-is suchthat the'two frequencies which are-rela tivelycontrolled, that is, thedifferencefrequency andthe frequency fr, arel not only-*maintained so las to have like values; that is, so that there is a zero difference frequency; buttha-tL these frequencies differ 'by only a very-small partofa cycle-so as to'be measurable,v for practicablepurposes. as a differencein phase rather than -as an integral frequency difference.- The controlis exertedat the very beginning of arelativephase displacement which initiatesA relative change of frequency; Because the control-is inthis-way effectedfso close to the-fountainhead, thesystem of-control is inherently VAmore-sensitive-and responsive than by conventional methods ofthe prior art. This phase detector isdisclosed indetailin Figs. 2Y and 3 and.- its operation; in Figs. A4 to 6 so that furtherv description will-be defer-red at this time.

The outputproduct -of the phase-.detector-is a direct-potential the-value of-whchA is a function of the relative difference of phase so as to be directly responsive to the initiation` ofr the-relative frequency` change. This potential is impressed on the frequency control circuit, which in turn-is usedto varythe frequency'of one-of the unit oscillators, oscillator No. 2 in the in-. stant case. In thefalternative-instance-ofthe use of a prime sourceas-the-controlled source, it would vary the frequency thereof'. In either case the variationinherently tends --to be insuch a direction as -to loe-compensatory, so as to tend to maintain the controlling and controlled frequencies in exactl equality, that is, in locked synchronism. In the practical case therewould be a slight phase din'erence between the waves, but the optimum-Y condition in practice cfa particu-V lar circuit of the invention-*has` been that in which the two waves are maintained so closely in synchronismthat therel is only'a small fraction of a cycle variationof this-phase difference.

While various means have-been suggested for achieving a frequency change by the use of. a controlling direct potential, applicant suggests a modification of a principle of so doinginvolving the use of a so-called reactance control tube which makes use -cf the principle that a change of potentialon a control` gridis expressible as a change of -shunt inductance of the-output circuit. Inthe present case the output circuit is connected across the frequency determining circuit of oscillator No. 2 so as to e`ectively1constitute a portion thereof, and, therefore, so that the variation of control electrodepotential achieves a variation infrequency of the-wave from said oscillator No. 2. By this electrical frequency control as distinguished from aconventional mechanical frequency control such as by-variation of a variable inductanceor capacity, a more sensitive and responsive control may be achieved, so as to more nearly realize the possibilities of the extreme sensitivity and responsiveness ofthe phase detector. The reactance control tube -feature ofthis system differs from that of* prior systems in that a parallel input arrangement of two reactance controltubes are used.- These are necessary because a push-pull tube oscillatorA $11'- cuitgisuseda That is; it-is necessaryl to change anunbalanced-to-ground direct potential to abal anced'V inductance change across the oscillator tubes;v The continuityof-the frequency control circuit, that is, the circuit utilizing the output ofthe detector toY control thefrequency of oscillator No. Zwill be describedin greater'detailfin ccnnectionwith Figi 7;

The conjugate input phase detector disclosed in Fig: 2 is so Vfunctionally labeled as to fairly Wellindicatethe continuityy of the circuit and the function of the parts. The principle of its operation, as alreadyV described in the above treatment of Fig; 1- may Well beexemplifiedl by the use ofa somewhat different continuity of circuit especialllyi'as to the tube element. In the form disclosed,the tube-effectively comprises two rectiner paths each involving anindividual anodeand the nearest cathode. Because Vof the conjugate connection to the two anodes, as shown,-of thecircuitsfrom the effective-50 kilocycle source,- that is; fromV the high `frequency detector and the standard or controlling fre quency source generatingva wave ofthe same frequency, the output potential, that is, the potential between the cathodes and across the b-ridgeconstituted by resistors R, will be a -direct potential having a value determinedrbythe phaserelation of-theinput-waves. When these waves-have a quadrature-relation, the resultant direct potential has zero value. If an initial condition is assumed where thephases have this relation (this,-o'f course, alsoimplying equal frequencies) any-deviation. therefrom, as representing an initiation of a relative change in frequency, would be reiected in the building up of a direct potential across this bridge. This potential is that used in the operation of the-frequency control circuit of Fig. 7 whose operation has been described briefly in the description of that element in the over-all circuit of Fig. l. The condenser C, thecapacitance of whichfin a practical instance was .1 uf., tends to short-circuit any` alternating potential surviving 'from those impressed onV the detector so as to better insurethat the eventual output Vis a simple direct potential` as is required for the eiective operattion of the subsequent frequency control circu1 The operation of this phase detector is best explained by the use of the graphicalv devices of Figs. 4 to 6 taken with Fig. 3 disclosing the equivalent circuit of Fig. 2. The labeling of Fig. 3 is different from that of Fig.v 2,..except for the duplication of the labeling of Vresistors R, since the labeling ofv Fig. 3 has in mind the particular requirement `of the graphical showings-of Figs. 4 to 6. The labeling of the arrow pointed lines in said Figs. 4 to 6,' which are somewhat analogous to=vectors, indicate the potentials between correspondingly labeled points in Fig, 3.V The correspondence of the vrespective elements of Figs. 2 and 3 should be quite evident. The transformers shown at the left, the .secondary windings of which are indicated by DE and FG, respectively, are the two conjugate input transformers of Fig. 2, the two rectiers, shown at the top and bottom, correspond to the rectier or detector paths of the tube of Fig. 2 and the resistances R are the bridge resistances in Fig. 2 similarly labeled. The other elements of Fig. 3

follows.

This `kind `of conjugate input phase detector has the Yadvantage of. being able to indicate phase balance to as close as .0l degree and at the same time beindependent of level changes in the two input potentials of about i5 decibel. In its operation let it rst be assumed that the inputs to the two transformers have a quadrature phase relation, so that the phase of one potential differs from that of the other by 90 degrees. If the input transformer, whose divided secondary windings are coils OD and OE, is assumed to be perfect, the potentials across these secondary coils are in opposite phase and the conditions are as represented in Fig. 4. Let it be assumed that the rectifiers are suchthat the rectified output current depends only on the amplitudes of the respective input potential. In such a case the amplitude of the potential between points DG as applied tothe upper rectier will give a proportional current Im through the upper resistor R.. In Fig. 4 the line Im is, therefore, shown in the direction the corresponding potential line would take if shown, and for simplicity its amplitude has been made to have the amplitude of that potential. Similarly, the amplitude of the current In resulting from the like potential as applied to the lower rectifier will be as indicated in Fig. 4. These currents Im and In are the currents which set up the difference of potential across the output circuit of the phase detector. This is the potential which would exist across the condenser C of Fig. 2. In practice the conditions represented by Fig. 4 could easily be achieved by the use of a phase shifter of conventional type placed in the input circuit of either transformer. In this case the current in meter Ip would be the arithmetical difference of the two component currents Im and In, this difference having zero value for the case assumed.v

Now let it be assumed that the angular relationship of the impressed voltages, assumed in the treatment of Fig. 4 is changed by an amount y as shown in Fig. 5. The remaining part of said Fig. 5 shows the resultant condition. Im no longer equals In and Ip is therefore no longer zero. It is to be noted that the rate of change of Im-In is very rapid at the condition of balance so that a small phase departure produces a relatively large meter current. Therefore, the circuit constitutes a very sensitive phase detector. Fig. 6 shows that the same balanced conditions prevail even though the input transformer connected to oscillator No. 3 is not perfect so that the voltage OD is not exactly 180 degrees displaced from the voltage OE, in the case represented by Fig. 6 differing therefrom by 2x degrees. It is also evident that changes in amplitude of the input potentials do not affect the difference Im-In for either of the conditions represented by Figs. 5 and 6 when the vector for the potential OG is symmetrically positioned with respect to the potentials OD and OE as would readily occur in practice. Instead of using the conjugate input transformers of Fig. 3 it is obvious that a Wheatstone bridge network comprised wholly of resistances could be used, the secondary windings of the input transformers being connected to different pairs of diagonally opposite points of the bridge. Such an arrangement might be of advantage since sometimes it is difficult to obtain coils having adequate midpoint balance.

Fig. 7 illustrates the frequency control circuit of Fig. 1 and its connection to the oscillator to be immediately controlled, that is, the oscillator No. 2. Said oscillator may be any conventional typeV of push-pull oscillator, the push-pull type of oscillator being assumed in the instant case because of its relatively large ,power output with a comparatively low harmonic content. The use of the particular control circuit disclosed is necessary in view of this particular type of oscillator, for reasons thatwill be given later. Inthe interest of simplicity, the oscillator is shown without the elaborations which would distinguish a commercial circuit of the same type. In general, it is of the type of the push-pull oscillator of U. S. patent to Hartley 1,472,470, October 30, 1923. The frequency determining circuit so labeled, is related to the push-pull connected tubes in a now well-known manner. The important consideration is that the frequency determining circuit is connected to the output circuit of the frequency control circuit shown at the right. This frequency control circuit is a combination of tubes 2 and 3 and their irnmediately associated circuits in parallel input relationship. The continuity of the circuit is believed to be obvious without specic description of parts as identified by label. The electrical Values disclosed are those for a typical circuit what has been found effective in practice.

The basic principle of operation of this circuit is that now quite commonly used for varying the tuning of a circuit by electrical, as distinguished from mechanical, means. The means involves the use of a reactance control tube, which means that a potential impressed on the control electrode of such tube determines to a great extent the input impedance of the said tube. Since said input potential, by reflection, effectively includes the impedance of the output circuit, or, from another point of view may be thought ci' as included therein, the ultimate result is that a tuned circuit connected to the output of the tube may be effectively adjusted in tuning by variation of the control electrode potential. VrI'he basic principle, as used in the synchronization control of an oscillator, and, therefore, somewhat analogously as in the present instance,V is well illustrated by U. S. patent to I-Iarper 2,066,528, January 5, 1937. Depending on the particular relation of electrical elements the output impedance change may be that of a capacitance or that of an inductance. In the instant case the change is an inductance change because of the circuits connecting the plates and control electrodes, although applicants invention as pertaining to this reactance control tube circuit does not depend on the circuits individual tothe respective tubes but to the push-pull relationship of such tubes. In order to control the push-pull oscillator assumed, by use of the reactance control tube principle, a balanced-toground reactance change is required. The parallel .input control tube circuit converts an impressed unbalanced direct kpotential to such balanced-to-ground reactance change. The parallel input controlcircuit,` therefore, adapts the operation'of the control principle used to the conditions'imposed by the choice of oscillator. The use of 'arsingle tube control circuit would result in a double ground on the oscillator circuit which would annul the effect of the control circuit and even the operation of the oscillator itself.

While the invention has been described in connection with a particular embodiment, certain variations have been suggested and it is to be understood that many additional variations are possible within the scope of the invention, as defined in the appended claims.

What is claimed is:

1. A system for maintaining the frequencies of two waves in locked frequency relation, comprising a source of controlled frequency waves including a reactive frequency determining circuit, a source of controlling frequency waves, a conjugate input phase detector for combining the said waves with the resultant production of a direct potential Varying as in accordance with the change in relative phase of the combined waves, and a reactance control tube circuit connecting said phase detector with said source of controlled waves for converting said changes in direct potential to corresponding changes in a reactance element of said frequency determining circuit, said source of controlled waves being a push-pull oscillator and said reactance control circuit comprising a pair of reactance control tubes in like parallel input relation, the output electrodes of said parallel input reactance tube circuit, between which the reactance change occurs, being connected to the frequency determining circuit of said push-pull oscillator so that its reactance is effectively a part thereof.

2. A system for maintaining the frequencies of two waves in locked frequency relation, comprising a source of controlled frequency waves including a reactive frequency determining circuit, a source of controlling frequency waves, a conjugate input phase detector for combining the said waves with the resultant production of a direct potential varying as in accordance with the change in relative phase of the combined waves, and a reactance control tube circuit connecting said phase detector with said source of controlled waves for converting said changes in direct potential to corresponding changes in a reactance element of said frequency determining circuit, said source of controlled waves being a push-pull oscillator in which the frequency determining circuit is connected between the output electrodes and said reactance control circuit comprising a pair of reactance control tubes in like parallel input relation, the output electrodes of said parallel input reactance control circuit, between which the reactance change occurs, being connected to said frequency determining circuit of said push-pull oscillator so that its reactance is effectively a part thereof.

3. A phase difference control system for controlling a given difference frequency wave as in accordance with the frequency of a controlling wave, comprising oscillators for generating twoV waves the difference between which is to be controlled, one of said oscillators comprising a reactive frequency determining circuit, a detector for combining the waves from said oscillator, a source of control waves, a conjugate input phase detector for combining the resultant difference frequency wave with the control wave whereby to obtain a direct potential variable responsively to changes in relative phase of the two waves impressed thereon, a reactance control tube circuit connected to said detector for converting said variable directpotential into a correspondingly variable inductance, and means connecting said control circuit with said frequency determining circuit whereby said variable reactance is effectively a part thereof, whereby the frequency of the oscillator, comprising said frequency determining circuit may vary in frequency compensatorily and so as to maintain the difference frequency of which the frequency of its wave is a component in locked synchronism with the Wave from the control oscillator.

4. The circuit specified in claim 3 in which the oscillator which comprises said frequency determining circuit is a push-pull oscillator and in which said reactance control tube circuit comprises a parallel input combination of two reactance control tubes, the output electrodes thereof, between which therefore the variable reactance occurs, being connected in symmetric relation to the frequency determining circuit of said push-pull oscillator.

5. The circuit specified in claim 3 in which the oscillator which comprises said frequency determining circuit is a push-pull oscillator having said frequency determining circuit connected across its output electrodes, and in which said reactance control tube circuit comprises a parallel input combination of two reactance control tubes, the output electrodes thereof, between which therefore the variable reactance occurs, being connected in symmetric relation to the frequency determining circuit of said push-pull oscillator.

ALLEN F. POMEROY. 

